U.S. patent number 6,459,709 [Application Number 09/774,904] was granted by the patent office on 2002-10-01 for wavelength-tunable semiconductor laser diode.
This patent grant is currently assigned to Nova Crystals, Inc.. Invention is credited to Steven Gregg Hummel, Chau-Hong Kuo, Chenting Lin, Yu-Hwa Lo, Mei-Ling Shek-Stefan, Sergey V. Zaytsev.
United States Patent |
6,459,709 |
Lo , et al. |
October 1, 2002 |
Wavelength-tunable semiconductor laser diode
Abstract
A wavelength-tunable distributed feedback (DFB) laser is
disclosed where the lasing wavelength can be adjusted by adjusting
the bias current of the laser diode. Since the output power of the
laser diode also changes with the bias current, a one-to-one
correspondence between the lasing wavelength and the output power
of the laser can be established. Consequently, the lasing
wavelength can be measured directly from the photocurrent of a
power monitoring detector facing the back-end of the laser diode.
This provides an extremely simple method for wavelength
monitoring.
Inventors: |
Lo; Yu-Hwa (San Diego, CA),
Hummel; Steven Gregg (Los Altos, CA), Lin; Chenting
(Poughkeepsi, NY), Kuo; Chau-Hong (Sunnyvale, CA),
Shek-Stefan; Mei-Ling (Sunnyvale, CA), Zaytsev; Sergey
V. (Cupertino, CA) |
Assignee: |
Nova Crystals, Inc. (San Jose,
CA)
|
Family
ID: |
25102638 |
Appl.
No.: |
09/774,904 |
Filed: |
January 31, 2001 |
Current U.S.
Class: |
372/20;
372/45.01 |
Current CPC
Class: |
B82Y
20/00 (20130101); H01S 5/0622 (20130101); H01S
5/1032 (20130101); H01S 5/3407 (20130101); H01S
5/3415 (20130101); H01S 5/3416 (20130101) |
Current International
Class: |
H01S
5/062 (20060101); H01S 5/00 (20060101); H01S
5/34 (20060101); H01S 5/10 (20060101); H01S
005/00 (); H01S 003/10 () |
Field of
Search: |
;372/20,45,50,46
;438/32 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
T L. Koch, U. Koren, and B. I. Miller, "High-performance tunable
1.5 .mu.m InGaAs/InGaAsP multiple quantum well distributed Bragg
reflector lasers", Applied Physics Letters, vol. 53, pp. 1036-1038,
1988. (Sep.1988). .
T. L. Koch and U. Koren, "Semiconductor photonic integreated
circuits", IEEE Journal of Quantum Electronics, vol. 27, pp.
641-653,1991. (Mar. 1991). .
M. Oberg, P.-J. Rigole, S. Nilsson, T. Klinga, L. Backbom, K.
Streubel, J. Wallin, and T. Kjellberg,"Complete single mode
wavelength coverage over 40 nm with a super structure grating DBR
laser", IEEE Journal of Lightwave Technology, vol. 13, pp.
1892-1897, 1995. (Oct.1995). .
B. Mason, J. Barton, G. A. Fish, L. A. Coldren, and S. P. DenBaars,
"Design of sampled grating DBR lasers with integrated semiconductor
optical amplifiers", IEEE Photonics Technologies Letters, vol. 12,
pp. 762-764, 2000. (Jul. 2000)..
|
Primary Examiner: Leung; Quyen
Attorney, Agent or Firm: Fish & Neave
Claims
We claim:
1. An electrically pumped semiconductor laser device having a
waveguide region, the waveguide region comprising an active layer
and a carrier reservoir, wherein electric carriers are injected
into the active layer by tunneling through a reversed biased p-n
junction disposed between the active layer and the carrier
reservoir to produce lasing radiation by recombination in the
active layer, and wherein a lasing wavelength of the laser device
is determined by a carrier concentration in the carrier reservoir,
with said carrier concentration depending on the pump current.
2. The laser device of claim 1, wherein the laser device is a
distributed feedback (DFB) laser.
3. The laser device of claim 1, wherein at least one of the active
layer and the carrier reservoir comprise a quantum well.
4. The laser device of claim 1, wherein a bandgap of the active
layer is smaller than a bandgap of the carrier reservoir.
5. The laser device of claim 1, wherein the waveguide comprises
InGaAlAs.
6. An electrically pumped wavelength-tunable semiconductor
distributed feedback (DFB) laser comprising: a first cladding layer
of a first conductivity type, a second cladding layer of a second
conductivity type, and an optical waveguide region disposed between
the first cladding layer and the second cladding layer, the optical
waveguide region comprising an active layer and a carrier
reservoir, wherein the active layer is electrically isolated from
the carrier reservoir by a reverse-biased p-n junction disposed
between the active layer and the carrier reservoir so as to retain
a concentration of electric carriers in the carrier reservoir
substantially independent of a laser drive current, with the
concentration of the electric carriers in the carrier reservoir
determining a refractive index of the optical waveguide region, and
a grating disposed proximate to the waveguide region and
determining in conjunction with the refractive index a lasing
wavelength, wherein the wavelength of the DFB laser can be
controllably tuned by adjusting a laser drive current.
7. The laser of claim 6, wherein the first and second cladding
layers comprise InP and the waveguide region comprises at least one
of AlGaInAs and InGaAsP.
8. A wavelength-tunable laser system comprising: an electrically
pumped semiconductor distributed feedback (DFB) laser producing a
laser beam and including a first cladding layer of a first
conductivity type, a second cladding layer of a second conductivity
type, an optical waveguide region disposed between the first
cladding layer and the second cladding layer, the optical waveguide
region comprising an active layer and a carrier reservoir, wherein
the active layer is electrically isolated from the carrier
reservoir by a reverse-biased p-n junction disposed between the
active layer and the carrier reservoir so as to retain a
concentration of electric carriers in the carrier reservoir
substantially independent of a laser drive current, with the
concentration of the electric carriers in the carrier reservoir
determining a refractive index of the optical waveguide region, and
a grating disposed proximate to the waveguide region and
determining in conjunction with the refractive index a lasing
wavelength, an optical amplifier receiving the laser beam and
producing an amplified laser beam, a first detector that measures
an output power of the laser beam, wherein the lasing wavelength is
determined by the measured output power, and a second detector that
measures an output power of the amplified laser beam, with the
output power of the amplified laser beam capable of being adjusted
independent of the lasing wavelength.
9. The laser system of claim 8, further comprising a modulator
which modulates the amplified laser beam in response to a
modulation signal applied to the modulator.
10. A method of producing wavelength-tunable laser radiation from a
DFB laser structure using a single pump current, comprising:
providing in a waveguide region of the DFB laser structure a
carrier reservoir that is electrically isolated from an active
layer by a reverse-biased p-n junction, adjusting the pump current
to change an index of refraction of the waveguide region through a
change in a carrier concentration in the carrier reservoir and an
output power of the DFB laser structure, and determining the
wavelength of the laser radiation from the output power.
11. The method of claim 10, wherein the wavelength of the laser
radiation is approximately 1.5 .mu.m.
12. The method of claim 10, wherein the DFB laser structure is made
of a material selected from the group consisting of InP, InGaAsP,
and GaAlInAs.
13. The method of claim 10, further including amplifying the laser
radiation so that an optical power of the amplified laser radiation
can be selected independent of the wavelength.
14. The method of claim 10, further including modulating the laser
radiation so that an optical power of the modulated laser radiation
can be selected independent of the wavelength.
Description
FIELD OF THE INVENTION
This invention relates to the field of wavelength-tunable
semiconductor lasers, and in particular to controllably tuning the
lasing wavelength by controlling the optical output power of the
laser.
BACKGROUND OF THE INVENTION
Wavelength-tunable lasers have found important applications in
optical communication and sensing. Wavelength-tunable lasers play a
central role in particular for dense wavelength division
multiplexing (DWDM) systems that form the backbone of today's
optical communication network. The term "wavelength-tunable laser"
is typically applied to a laser diode whose wavelength can be
varied in a controlled manner while operating at a fixed heat sink
temperature. At the 1550 nm wavelength regime on which most DWDM
systems operate, a wavelength shift of 0.1 nm corresponds to a
frequency shift of about 12.6 GHz. At a given heat sink
temperature, the central wavelength of a conventional distributed
feedback (DFB) laser diode may be red-shifted by as much as 0.3 nm
or about 40 GHz due to the rise in the temperature of the junction
by Ohmic losses. In contrast, at a given heat sink temperature, the
wavelength of a tunable laser may vary by several nanometers,
corresponding to hundreds or even thousands of GHz, covering
several wavelength channels on the International Telecommunication
Union (ITU) grid. Depending on the physical mechanisms of
wavelength tuning, the lasing wavelength can be tuned in either
positive (red) or negative (blue) direction. Controlled wavelength
tunability offers many advantages over conventional fixed
wavelength DFB lasers for DWDM operation. It enables advanced
all-optical communication networks as opposed to today's network
where optics is mainly used for transmission and the network
intelligence is performed in the electronic domain. All-optical
networks can eliminate unnecessary E/O and O/E transitions and
electronic speed bottlenecks to potentially achieve very
significant performance and cost benefits. In addition, a less
extensive inventory of wavelength-tunable lasers than of laser with
a fixed wavelength is required. Keeping a large inventory of lasers
for each and every wavelength channel can become a major cost
issue. For advanced DWDM systems, the channel spacing can be as
narrow as 50 GHz (or about 0.4 nm in wavelength), with as many as
200 optical channels occupying a wavelength range of about 80 nm.
For the reasons stated above, wavelength-tunable lasers have
attracted considerable interest in optoelectronic device
research.
There exist different design principles for tunable lasers. Almost
all wavelength-tunable laser designs make use of either the change
of refractive indices of semiconductor or the change of laser
cavity length to achieve wavelength tuning. For the former, common
mechanisms for index change include thermal tuning, carrier density
tuning (a combination of plasma effect, band-filling effect, and
bandgap shrinkage effect), electro-optic tuning (linear or
quadratic effect), and electrorefractive tuning (Franz-Keldysh or
quantum confined Stark effect). For DFB lasers, the wavelength of
the laser light propagating in the waveguide is basically
determined by the grating period .LAMBDA.. The free-space lasing
wavelength .lambda. is given by .lambda.=2 n.sub.eff .LAMBDA.,
where n.sub.eff is the effective index of refraction of the
waveguide and .LAMBDA. is the period for first-order gratings.
Accordingly, the change .DELTA..lambda. in the lasing wavelength
.lambda. is directly proportional to the change .DELTA.n of the
index of refraction n.sub.eff.
Referring to FIG. 1, a prior art three-section DBR tunable laser
100 includes an optical gain section 101, a phase control section
102, and a tunable DBR section 103. A first current source 104
pumps the gain section 102 to generate optical gain; a second
current source 105 injects carriers to adjust the phase condition
of the phase control section 102 so that the resonant frequency
matches approximately the peak of the DBR reflectivity; and a third
current source 106 controls the reflectivity peak by changing the
effective index n.sub.eff of the Bragg waveguide section 103. With
proper selection of the currents in the DBR region 103 and in the
phase control region 102, quasi-continuous wavelength tuning can be
achieved. All three sections 101, 102, 103 are optically connected
to minimize residue reflections and coupling loss; however, the
sections 101, 102, 103 have to be electrically isolated from one
another, for example, by layers 107 disposed between the respective
sections 101, 102, 103. Three currents, responsible for the gain
region, DBR region, and phase control region, have to be supplied;
and the lasing wavelength depends on all three currents and is
particularly sensitive to the currents in the DBR and phase control
region. A continuous wavelength tuning range of about 10 nm can be
achieved using this design.
Modifications of the three-section DBR lasers include sampled
grated four-section DBR lasers and vernier-tuning sampled grating
DBR lasers (not shown). The last device requires four separately
controlled current sources to achieve the full tuning range (about
80 nm quasi-continuous tuning).
Alternatively, the lasing wavelength can also be changed by
changing the physical length of laser cavity in the surface normal
direction. This mechanism has been applied, for example, to
vertical-cavity surface-emitting lasers (VCSELs) where typically
due to the short cavity length only one or at most very few lasing
modes fall within the gain peak. Referring to FIG. 2, a prior art
wavelength-tunable VCSEL structure 200 is based on surface
micromachining technology. The laser device 200 includes a bottom
dielectric DBR mirror 202, a top dielectric DBR mirror 201, an
electrostatically controlled membrane 203, and an active region
204. Electrically pumped micro-electro-mechanically tuned VCSEL in
the 1550 nm wavelength regime have not yet been demonstrated.
However, the laser device 200 can be optically pumped by an
incoming pump beam 205 (e.g. a beam from a 980 nm wavelength pump
laser) through the bottom mirror 202, with the laser output 206
being emitted from the top mirror 201 disposed on the membrane 203.
Wavelength-tuning is obtained by changing the cavity length of the
VCSEL through the movement of the membrane 203. With a surface
micromachined tunable mirror, a continuous tuning range of 40 nm
has been demonstrated with an output power of up to 7 mW coupled to
a single mode fiber.
Multiple-section DFB lasers in general have a smaller tuning range
than multiple-section DBR lasers, except for the tunable twin-guide
(TTG) DFB lasers where relatively wide (about 6 nm) and continuous
tuning can be achieved.
In DWDM systems, the wavelength of the channel has to be stabilized
within a few gigahertz from the ITU grid, typically less than 10%
of the channel spacing. A change of the junction temperature and/or
device degradation can cause wavelength drift beyond its acceptable
range. Achieving wavelength stability requires monitoring the
wavelength in real time using a sophisticated feedback mechanism.
Several commercially available devices and their operation for
accurately monitoring the laser emission wavelength are shown in
FIGS. 3, 4, and 5. Common to these devices is an optical
interference device such as a Fabry-Perot etalon placed between the
laser and a photodetector. Critical for the device performance are
the mechanical stability and angular precision of the etalon and
the collimation of the laser beam impinging on the etalon.
Referring now to FIG. 3, a wavelength-monitoring system 300
includes an optical beam splitter 301, a Fabry-Perot (F-P) etalon
302 connected to a first output of the beam splitter 301, a first
photodetector (PD) 303 following the F-P etalon 302, with a second
PD 304 connected to the second output of the beam splitter 301 as a
reference detector. Once the system is calibrated, the lasing
wavelength can be determined from the ratio of the photocurrents of
the PDs 303, 304, as illustrated in FIG. 4. In the illustrated
example showing exemplary target wavelengths .lambda..sub.n,
.lambda..sub.n+1, a rise in the ratio I.sub.1 /I.sub.2 of the
measured photocurren would indicate a decreasing lasing wavelength,
while a decrease in the ratio I.sub.1 /I.sub.2 of the measured
photocurrents would indicate an increasing lasing wavelength. Note
that there exist multiple wavelengths .lambda..sub.n,
.lambda..sub.n+1, that can yield the correct photocurrent ratio,
and each wavelength may correspond to an ITU wavelength channel.
This design has problems with generating a proper error signal when
wavelength hopping occurs.
FIG. 5 shows a more detailed design of a commercial wavelength
monitoring system 400, excluding electronic circuitry. A small
fraction, typically a few percent, of the received laser light is
coupled into the monitoring system by an optical power splitter
401. The beam is collimated in collimator 402 and then split into
two approximately equal signals by a beam splitter 403. The
photocurrent of PD 404 provides the reference signal proportional
to the power of the received laser light. The photocurrent of PD
406 is related to the received power being transmitted through the
Fabry-Perot etalon 405. The ratio of these two photocurrents does
not depend on the output power of the received laser light.
The manufacturing and operating complexity of the
wavelength-tunable lasers and the wavelength monitoring system
represent barriers for the production of low cost
wavelength-tunable laser modules for low-cost DWDM systems suitable
for metropolitan area networks (MAN). It would therefore be
desirable to provide a new design for a wavelength-tunable laser
where the lasing wavelength can be tuned by a single current source
and the lasing wavelength can be measured without requiring
interferometric devices.
SUMMARY OF THE INVENTION
According to one aspect of the invention, a wavelength-tunable
distributed feedback (DFB) laser structure is disclosed where the
lasing wavelength can be adjusted by adjusting a single bias
current of the laser diode. Since the output power of the laser
diode also increases with the bias current, one can establish a
straightforward, one-to-one correspondence between the lasing
wavelength and the output power of the laser. Consequently, the
lasing wavelength can be measured directly by a power monitoring
detector facing, for example, the back-end of the laser diode.
To provide wavelength-tuning, the DFB laser structure includes a
second set of quantum wells or a waveguide layer next to the lasing
quantum wells as "carrier reservoir". The second set of quantum
wells or the waveguide layer has to meet several requirements in
order to function effectively as a carrier reservoir without
adversely affecting the laser performance. First of all, the
carrier reservoir has to have a higher bandgap than the lasing
quantum wells to minimize the optical loss. Secondly, a carrier
propagation barrier needs to be present between the lasing quantum
wells and the reservoir to avoid carriers falling into the lasing
quantum wells, which have the lowest bandgap of all materials and
the strongest tendency of attracting carriers. Thirdly, the carrier
reservoir has to be located in a region where the intensity of
optical field is significant so the carrier induced index change
can contribute to the change of lasing wavelength of a DFB laser.
Finally, the presence of the carrier reservoir should not trigger
high-order transverse modes. In other words, the laser should
operate in a single longitudinal mode and single spatial mode. A
structure meeting the above requirements includes a reverse-biased
tunnel junction made of heavily doped p.sup.+ - and n.sup.30
-layers disposed between two sets of quantum wells to prevent
carrier leakage from the reservoir back to the active quantum
wells. Because of the carrier tunneling effect, holes can tunnel
through the n.sup.+ /p.sup.+ junction and reach the carrier
reservoir to meet the electrons. The carrier concentration in the
reservoir is then determined by the spontaneous emission rate and
Auger recombination rate. This follows approximately the empirical
equation I=BN.sup.2 +CN.sup.3 where I is the current, N the carrier
concentration in the reservoir, and B and C the rates for
spontaneous and Auger recombination, respectively. Contributions
from defect-related recombination, which is linearly proportional
to the carrier concentration, are neglected.
Embodiments of the invention may include one or more of the
following features. The laser can be grown on an n-InP substrate
and have a p-InP as the upper cladding layer. A thin layer of
material having a higher refractive index than InP can be
introduced to form gratings for index-coupled DFB lasers. Two
unintentionally doped graded-index (GRIN) confinement regions can
be located on either side of the quantum well active layers to
provide carrier and optical confinement. Between the two GRIN
layers, the quantum wells forming the active layer and responsible
for lasing are positioned closer to the p-InP cladding layer and
additional quantum wells having a higher ground state energy than
the active layer and forming the carrier reservoir are located near
the n-InP lower cladding layer.
According to another aspect of the invention, the laser output may
be coupled to a semiconductor laser amplifier (SLA) to allow an
independent adjustment of the lasing wavelength and the optical
output power of the device. Optionally, the laser output may be
coupled to an optical modulator, such as an electro-absorption
modulator, to externally modulate the laser light to reduce
chirping. The modulator may be used with or without the SLA.
Detectors may be provided to measure an output power of the laser
beam and/or the amplified laser beam and/or the modulated laser
beam.
Further features and advantages of the present invention will be
apparent from the following description of preferred embodiments
and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The following figures depict certain illustrative embodiments of
the invention in which like reference numerals refer to like
elements. These depicted embodiments are to be understood as
illustrative of the invention and not as limiting in any way.
FIG. 1 is a schematic diagram of a conventional 3-section DBR
tunable laser,
FIG. 2 is a schematic diagram of an optically pumped MEMS VCSEL
tunable laser,
FIG. 3 is a simplified block diagram of a conventional wavelength
monitor,
FIG. 4 shows schematically the operation of the wavelength monitor
of FIG. 3,
FIG. 5 shows the wavelength monitor of FIG. 3 in greater
detail,
FIG. 6 is a schematic diagram of a simplified epitaxial layer
structure for a wavelength-tunable distributed feedback (DFB) laser
of the invention,
FIG. 7 shows an energy band diagram of the laser structure of FIG.
6 under an applied forward bias V.sub.a,
FIGS. 8a-c shows the laser characteristics for (a) wavelength
versus drive current, (b) optical output power versus drive
current, and (c) wavelength versus optical output power,
FIG. 9 shows a wavelength-tunable laser integrated with an optical
amplifier for simplified wavelength monitoring and an optional
external optical modulator.
DETAILED DESCRIPTION OF CERTAIN ILLUSTRATED EMBODIMENTS
One aspect of the present invention relates to a semiconductor
laser with a novel epitaxial layer structure, wherein the laser can
be wavelength-tuned by varying the pump power of the laser using a
single current source. Another aspect of the present invention
relates to a simplified arrangement for measuring the lasing
wavelength without employing interferometric devices.
A major challenge of using a single current source to tune the
wavelength is carrier pinning effect above threshold. In
semiconductor diode lasers having a conventional active layer
consisting of a single waveguide or single/multiple quantum wells,
the carrier concentration in the waveguide/quantum wells increases
with the injected current below the lasing threshold, with the
carrier concentration becoming essentially pinned at a fixed value
once the threshold current is reached. Beyond threshold, the laser
output power increases with the current linearly, but the carrier
concentration, hence the effective index of refraction, remains
more or less constant. Because of the carrier pinning effect, the
contribution of carrier-induced index change becomes negligible. A
solution provides by the present invention incorporates a second
set of quantum wells or a second waveguide layer next to the lasing
quantum wells as "carrier reservoir". The second set of quantum
wells or a second waveguide layer is electrically isolated from the
active quantum wells by a tunnel junction made of heavily doped
p.sup.+ - and n.sup.+ -layers. When one applies a forward current
to the laser structure, the laser active junction is forward biased
as regular laser p/n junction and electron-hole recombination
occurs in the quantum wells. However, the n.sup.+ /p.sup.+ tunnel
junction that separates two groups of quantum wells is
reverse-biased.
FIG. 6 shows an exemplary epitaxial layer structure 600 according
to the invention that provides the tunability. In all other
aspects, the device is similar to a conventional distributed
feedback (DFB) laser, with a grating similar to that shown for the
DBR section 103 of FIG. 1. The layer structure 600 is fabricated on
a semiconductor substrate 601 and includes, in that order and
starting from the substrate 600, a lower cladding layer 602, a
lower graded-index (GRIN) layer 603, a carrier reservoir layer 604,
a tunnel junction 605, an active layer 606, an upper GRIN layer
607, and an upper cladding layer 608. Other typical laser device
layers, such as buffer layers and/or contact layers, are not shown.
Also not shown is the grating layer of the DFB laser structure 600
which can be placed either below or above the active layer. The
cladding layers 602, 608 and the GRIN layers 603, 607 have a higher
bandgap and a lower refractive index than the active layer 606 to
assist in the confinement of carriers and optical field in the
active region. The active layer 606 produces optical gain and
roughly determines the lasing wavelength within the gain profile.
The gain profile is typically about 100 nanometers wide. The active
layer may be a "bulk" semiconductor layer or one or more quantum
wells.
Unlike a conventional epitaxial semiconductor laser device
structure which typically consists of layers 602, 603, 606, 607,
608 grown on substrate 601, the invented structure includes in
addition the carrier reservoir layer 604 and the tunnel junction
layer 605. The function of the latter two additional layers, the
carrier reservoir layer 604 and the tunnel junction layer 605, will
now be described.
Referring now to FIG. 7, from right to left is shown a schematic
energy band diagram 700 of the laser structure 600, with the
reference numerals referring to the respective layers of laser
structure 600. The quasi Fermi level 708 shows that the voltage
drop occurs at the laser active p-n junction and the carrier
reservoir p-n junction. These two junctions are stacked together
and separately by a tunnel junction having a negligible voltage
drop, as shown in the energy and diagram. The band diagram 700 can
apply to any material system known in the art for fabricating
semiconductor quantum-well heterojunction laser structures, such as
GaAlAs, InP/GaInAs, GaAlInN and the like. In an exemplary structure
for a 1550 nm wavelength laser emission, the following composition
and doping could be employed: substrate 601: n-InP; lower cladding
layer 602: n-InP; lower graded-index (GRIN) layer 603: n-InGaAlAs;
carrier reservoir layer 604: nominally undoped InGaAlAs MQW; tunnel
junction 605: n.sup.+ /p.sup.+ InGaAsP or InGaAlAs having a bandgap
wavelength of around 1.4 .mu.m; active layer 606: nominally undoped
InGaAIAs MQW; upper GRIN layer 607: p-InGaAlAs; and upper cladding
layer 608: p-InP. The InGaAsP and InGaAlAs layers are preferably
lattice-matched to InP and are understood to have the composition
In.sub.x (Ga.sub.y Al.sub.1-y).sub.1-x As, wherein x determines the
lattice-match to the InP substrate and the bandgap can be varied
for a constant x by varying y. The effective bandgap of the carrier
reservoir 604 is selected to be lower than the bandgap of the
surrounding layers 603, 605 to form a "valley" to collect carriers
(both electrons and holes). However, the effective bandgap of the
carrier reservoir 604 is selected to be a slightly higher than the
active region to minimize the optical loss in the laser waveguide
610 which is formed by all the layers 603, 604, 605, 606 and 607.
The refractive index of the reservoir layer 604 decreases with the
number of carriers in layer 604. This causes a reduction
.DELTA.n.sub.eff of the effective index n.sub.eff of the laser
waveguide 610, leading to a decrease .DELTA..lambda. in the lasing
wavelength .lambda. according to the relation
.DELTA..lambda.=2.DELTA.*.DELTA.n.sub.eff. .LAMBDA. is the grating
period of the DFB laser, as described above.
However, for the carrier reservoir 604 to function properly,
several conditions have to be fulfilled: (1) the carrier reservoir
604 has to retain a portion of the injected carriers, with the
portion being related to, for example, proportional to the total
number of injected carriers, i.e., the operating current of the
laser; (2) the carrier reservoir 604 should be electrically
isolated from the active region 606, while optically being a part
of the active region 606; and (3) the carrier reservoir itself
should not contribute to lasing, i.e., the gain peak of the
reservoir 604 should be outside the operating wavelength of the DFB
laser.
When a laser is operated above threshold, the quasi-Fermi level and
the carrier concentration in the active region 606 are
approximately "fixed". An increase in the injection current
converts more electron-hole pairs into photons to generate higher
optical power without significantly changing the carrier
concentration in the active region 606 or lasing wavelength, except
for band filling. If the carrier reservoir 604 is not electrically
separated from the active region 606, the carrier concentration in
the reservoir 604 will also be roughly fixed, independent of the
injection current. The carrier concentrations in both regions 604,
606 can be decoupled by introducing a tunnel junction between the
active region 606 and the carrier reservoir 604. In this case,
above threshold, the carrier concentration in the reservoir 604
increases with increasing current, while the carrier concentration
in the active region 606 remains fixed. Since, as mentioned above,
n.sub.eff decreases with increasing carrier concentration in the
reservoir 604, the lasing wavelength of a DFB laser can be tuned by
adjusting the carrier density in the reservoir 604, i.e., the laser
drive current. In other words, the lasing wavelength controllably
decreases with increasing injection current and hence also with
increasing optical power. Because a unique relation exists between
the output power and the lasing wavelength, the wavelength of the
laser can be measured and adjusted simply by measuring the output
power of the laser without requiring sophisticated wavelength
monitoring devices.
The epitaxial layer structure is fabricated in a conventional
manner, for example, by MOCVD or MBE, so that the gain profile
covers the intended operation wavelength range and the layer and
device structure favor operation in the fundamental transverse
mode.
The coupling coefficient .kappa. of the DFB grating on one of the
layers close to the active region is selected so that the coupling
is in the range of 30 to 300 cm.sup.-1 and the product of .kappa.L
is between 1 and 10, where L is the laser cavity length.
For an operating wavelength of 1.5 .mu.m, the bandgap of the
carrier reservoir should be approximately 0.1 eV greater than the
bandgap of the active region. For this difference in bandgap, the
change in the refractive index with carrier concentration in the
reservoir is approximately dn/dN=-1.8.times.10.sup.20 cm.sup.-3,
and approximately dn/dN=-2.4.times.10.sup.20 cm.sup.-3 when the
difference in bandgap is reduced to 0.05 eV. The approximate
wavelength tuning range is given by .DELTA..mu.=.eta.
(dn/dN*.DELTA.N)*.lambda./n, wherein .eta. is the confinement
factor for the reservoir layer, .DELTA.N is the increase in the
carrier concentration variation with current above the lasing
threshold, and n is the effective index. Using typical parameters
of .eta.=0.2, dn/dN=-2.4.times.10.sup.20 cm.sup.-3,
.DELTA.N=3.times.10.sup.18 cm-3, n=3.3, and .lambda.=1550 nm, a
continuous wavelength tuning range .DELTA..lambda. of about 7 nm is
obtained. This is similar to the 6nm tuning range of twin-guide
(TTG) DFB lasers also using carrier induced index change for
wavelength tuning.
The expected wavelength tuning range is eventually limited by
junction heating, carrier-induced optical losses, and carrier
recombination in the reservoir.
FIGS. 8a-c show schematically characteristic curves for wavelength
versus current (FIG. 8a), optical power versus current (FIG. 8b),
and wavelength versus optical power (FIG. 8c) of the tunable DFB
laser of the invention. As evident from FIG. 8c, the wavelength can
be tuned by adjusting a single current and monitoring by the laser
output power without requiring a Fabry-Perot etalon. For DWDM
systems of 50 GHz channel spacing, the wavelength control has to be
within .+-.5 GHz (or .+-.0.4 .ANG.). If the tuning range of a laser
diode is 7 nm, one needs to measure the photocurrent to an accuracy
of 0.4/70, which requires an 8 bit resolution A/D converter. This
requirement can be easily met with low cost commercial A/D
converters having 14-bit resolution. The shot noise of the detector
is not expected to be an issue either since the wavelength
monitoring detector operates at a very low bandwidth.
According to another embodiment depicted in FIG. 9, the wavelength
of a laser system 900 can be adjusted independent of the output
power produced by the system 900. The system 900 includes the
wavelength-tunable DFB laser 901 of the type described above in
combination with an optical amplifier 902. The laser/amplifier
combination 900 may be, for example, a tunable laser monolithically
integrated with semiconductor optical amplifier or a hybrid
integration of the tunable laser with a fiber amplifier. The laser
system 900 further includes a back-end detector 903 for monitoring
the laser power, which in this case corresponds to the lasing
wavelength; focusing optics 904; a power splitter 905 receiving
light from the front end of the optical amplifier 902, with a
predetermined fraction of the received light split off and entering
an output power detector 906 to monitor the power coupled, for
example, into an optical fiber 907. Also shown in FIG. 9 is an
optional external modulator 908. Again; the wavelength of the laser
can be controlled by controlling only the laser drive current with
a single current source, while both the wavelength and the final
optical power into the fiber are monitored only by photodetectors
without interferometric components that are sensitive to
misalignment.
To modulate the light, the laser may be modulated either directly
by controlling the drive current or externally using an external
modulator. The preferred method of modulation depends on
applications. For directly detected (non-coherent) DWDM systems,
low chirping (wavelength/frequency variation with power) is
important to minimize dispersion penalty, so external modulation is
desirable. The external modulator 908 can be an electro-absorption
(EA) modulator or an interference-type electro-optic (EO)
modulator. The position of the modulator 908 and the optical
amplifier 902 can be interchanged although the arrangement shown in
FIG. 9 is more convenient from device fabrication and signal
isolation point of view. On the other hand, if coherent detection
systems such as homodyne and heterodyne systems are used, direct
modulation of tunable lasers might be preferred. The laser of the
invention has an optical FM efficiency that is approximately 100
times that of conventional DFB lasers (about 30 GHz/mA compared to
about 300 MHz/mA for conventional DFB lasers). This means that the
modulation current can be as much as 30 times smaller in an optical
frequency division multiplexing (OFDM) or optical
frequency-shift-keying (FSK) system, making the laser of the
invention more efficient than the conventional DFB laser.
While the invention has been disclosed in connection with the
preferred embodiments shown and described in detail, various
modifications and improvements thereon will become readily apparent
to those skilled in the art. Accordingly, the spirit and scope of
the present invention is to be limited only by the following
claims.
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